The present invention relates to an oscillator for use in a preconditioner comprising a rectifier (1), a converter (2) receiving an input voltage (Uin) and supplying an output voltage (Uout), and a control unit (3) for controlling the peak current mode to maintain a constant output voltage. The invention also relates to a voltage converter provided with such an oscillator, and a preconditioner with such an oscillator.
A simple way to rectify an AC source to a DC output is to use a diode bridge rectifier and a capacitor. However, when the input power is greater than 75 W, this is not allowed according to the requirements of IEC1000-3-2.
Alternatively, a Switch Mode Power Supply (SMPS) can be used, comprising e.g. a diode bridge rectifier and a boost converter, to control the current drawn from the mains. Although it is possible to draw a sine-shaped current from the mains (e.g. using a Power Factor Control (PFC)), square wave current or a trapezoid-shaped current may also be used, at least under certain power levels. An advantage of using a square wave input current is that the peak value is 1.27 lower than a sine-shaped current for the same output power. A lower peak level means that a smaller inductive component (coil) can be used, reducing cost and space requirements.
An example of a rectifier in combination with a boost converter drawing a trapezoid input current is given in U.S. Pat. No. 5,615,098, where the main object is to obtain a simpler control electronics. However, this converter operates in the continuous mode. Boost converters operating in the discontinuous mode are also well known in the art. In such converters, during every switching period, the current in the boost coil reaches a certain maximum and at the end is zero again (triangle form). Such converters have smaller coils compared to continuous mode converters, lower turn-on losses and emission levels due to the zero current at switching, and control of output is simpler.
An object of the present invention is to provide improved frequency control for a power converter operating in the discontinuous mode with peak current mode control.
A second object of the invention is to reduce the required size of the boost coil in a boost converter.
A third object of the invention is to provide a converter drawing a square wave or trapezoid-shaped input current.
According to a first aspect of the invention, these and other objects are accomplished by an oscillator of the type mentioned by way of introduction, having the features described in the independent claim 1.
This inventive switching frequency control scheme makes it possible to provide a converter operating in the discontinuous mode that draws a square wave input current. An important detail of this aspect of the invention is that both input and output voltages are allowed to influence the switching frequency. The function of the switching period (Tper) in accordance with the invention enables a better control of the input current supplied to the converter and, in particular, an essentially constant average input current to be obtained.
According to another aspect of the invention, the above switching period is realized substantially by an oscillator of the type mentioned by way of introduction, having an output signal dependent upon the voltage across a timing capacitor, and comprising a switching element arranged to connect the capacitor to ground when said output signal is low, thereby causing discharging of said capacitor during a discharge time. The oscillator further comprises a zener diode and a transistor, connected to the input voltage in such a way that, when the input voltage exceeds the zener voltage of the zener diode, a current flows through the transistor, thereby decreasing the discharge current and increasing the discharge time.
In this oscillator, the discharge current is divided into two regions, depending on whether the input voltage is lower or higher than the zener voltage.
This circuit can be realized by connecting the transistor emitter to the input voltage via a resistance, connecting the transistor collector to the anode of a diode, the cathode of which is connected to the capacitor, and connecting the transistor base to the cathode of the zener diode, the anode of which is connected to ground.
According to a preferred embodiment, a second zener diode, connected to the input voltage in such a way that, when the input voltage exceeds a voltage limit, equal to the sum of the zener voltage of the first zener diode and a function of the zener voltage of the second zener diode, the current through the transistor increases, thereby further decreasing the discharge current and increasing the discharge time.
The discharge current is now divided into three regions, (1) below the first zener voltage, (2) between the zener voltage and the voltage limit, and (3) above the voltage limit.
This circuit can be realized by connecting the cathode of the second zener diode to the input voltage via a resistance, and connecting the anode of the second zener diode to the cathode of the first zener diode and to the base of the transistor.
The oscillator preferably comprises a second switching element arranged to connect the collector of the transistor to ground when said output signal is high.
Other aspects of the invention include a converter and a preconditioner comprising such an oscillator.